This invention is directed to a method of hydraulically fracturing a subterranean earth formation from a well penetrating the formation and more particularly to sealing the upper edge of a proppant pack or bed formed in the lower portion of a vertical fracture to better extend the vertical fracture further into the formation and prop the extended portion thereof.
Hydraulic fracturing techniques are commonly employed in completing a well to increase the recovery of hydrocarbons from subterranean formations. Generally, these techniques involve injecting a fracturing fluid down a well and into the formation under sufficiently high pressure to initiate and propagate a fracture into the formation. Proppant materials are generally entrained in the fracturing fluid and are deposited in the fracture to maintain the fracture open. It is generally considered that at depths greater than about 3,000 feet, vertical or vertically inclined fractures rather than horizontal fractures are usually formed when carrying out hydraulic fracturing techniques. This is because at any substantial depths most formations have a preferred vertical fracture orientation because of naturally occurring planes of weakness in the formation and the fractures are formed and propagated along these planes of weakness.
Many different types of fracturing fluids have been employed. In an article entitled "What to Learn About Hydraulic Fracturing Fluids", T. C. Buchley and D. L. Lord, OIL AND GAS JOURNAL, Sept. 17, 1973, pp. 84-88, it is said that present day fracturing fluids are classed as Newtonian polymer solutions, cross-linked polymer solutions, emulsions, micellar solutions, and gel-organic liquids in solution with a liquefied gas. A fracturing fluid also normally contains propping material and fluid loss additives. A common propping material is sand of 20-40 mesh size. Other types of propping material sometimes used include glass beads and nut shells. The term "sand" is often used generically to include any propping material usable for propping or maintaining open fractures formed in subterranean formations by hydraulic fracturing techniques.
In the above-cited article to Buchley et al., it is taught that created fracture volumes are greatly diminished if a large portion of the injected fluid is lost to porous fracture faces. Thus, if a fluid does not inherently have satisfactory fluid loss control, it is generally necessary to add particulate and/or gelatinous material which forms a filter cake on fracture faces to control loss to the porous rock matrix. In U.S. Pat. No. 3,710,865 to Othar M. Kiel, it is pointed out that it has long been known that the fluid efficiency of a fracturing fluid must be high if fractures of reasonable length and widths are to be obtained. Fluid efficiency as used in fracturing operations is defined as the per cent of injected fluid which remains within the fracture and is a measure of the fluid loss characteristics of the fluid. Many fluids tend to leak off rapidly into the formation matrix and therefore provide low fluid efficiencies. The fluid efficiency can be improved by the addition of fluid loss control additives.
Fracturing fluids are sometimes designed, however, to be free of fluid loss additives and to have high leak-off properties. In U.S. Pat. No. 3,858,658 to Malcolm K. Strubhar et al., such fracturing fluids are found to be desirable. The invention described by Strubhar et al. is concerned with a hydraulic fracturing method for forming long, narrow, vertical fractures in a thick low permeability formation. The formation is treated by injecting thereinto in a single stage and at a relatively low injection rate a very large volume of relatively low viscosity fracturing fluid.
In carrying out hydraulic fracturing techniques, a formation is fractured or parted by application of hydraulic pressure. After release of the hydraulic pressure, the formation tends to close together again and thus heal the fracture. Propping material and generally sand have long been included in hydraulic fracturing fluids such that the propping material is deposited in the formed fracture to maintain the fracture open after the hydraulic pressure is released. Many efforts have been made to arrive at optimum techniques for placing proppants in fractures to obtain efficient flow channels. In U.S. Pat. No. 3,896,877 to Thomas C. Vogt, Jr. et al., there is described a hydraulic fracturing process of forming and propping a vertical fracture in a subterranean formation. Propping material is scheduled into the fracturing fluid in amounts sufficient to fill the fracture to a predetermined height concomitantly with the forming of the fracture. The schedule provides for a high concentration of propping material in the fracturing fluid early in the fracturing treatment and decreasing concentration as the fracture is extended into the formation. In U.S. Pat. No. 3,709,300 to David S. Pye, there is described a process for hydraulically fracturing a permeable subterranean formation traversed by a well in which the deposition of propping agent in the fracture adjacent to the well is assured. The fracture is formed and extended and an initial deposit of propping agent placed in the fracture using a low fluid loss fracturing fluid so that the permeability of exposed faces of the fracture is reduced. The permeability of the formation is then restored and additional propping agent is again deposited in the fracture using a fluid having a high fluid loss characteristic. A large proportion of the propping agent injected into the fracture in this stage of the treatment is deposited in the fracture adjacent to the well thereby assuring fracture permeability in the critical area adjacent to the well.
The use of fluid loss additives in fracturing fluids, though offering certain benefits, is also known to cause problems. For example, the fluid loss additives are deposited on the fracture faces, thus reducing the permeability of the subterranean formation adjacent the fracture. Subsequent treatments are called for to remove the fluid loss additives from the fracture faces. Fluid loss additives are also deposited in the proppant pack itself, thereby reducing the permeability of the proppant pack and thus reducing the flow capacity of the fracture.
Other problems have occurred when using fracturing fluids having little or no fluid loss additives in that a large amount of the fracturing fluid leaks off into the formation and thus limits the length of a fracture which may be formed in a formation. At times it is considered that the loss of fluid is so severe that a "sand out" occurs, that is, that the sand or proppant carried in the fracturing fluid builds up in the fracture to such an extent that it blocks flow of the fracturing fluid into the fracture. Such problems have occurred particularly when attempting to carry out a very large fracturing treatment using fracturing fluids having no fluid loss additives therein, and when refracturing wells in which a fracture has previously been formed and a proppant bed deposited therein. This invention provides a technique for mitigating such problems.